This invention relates to an improved process for the preparation of melamine by the conversion of urea and/or thermal decomposition products of urea wherein ammonia and carbon dioxide resulting from the melamine preparation are reprocessed to form urea. In a known process, this ammonia and carbon dioxide is condensed at a pressure of between about 0.5 and 70 bar to form an aqueous ammonium carbamate solution, whereafter the solution is brought to a pressure of at least about 100 bar and urea synthesis temperature whereupon the ammonium carbamate is at least in part converted into urea and water.
Such a process is described in U.S. Pat. No. 3,544,628 wherein the gaseous mixture obtained in the preparation of melamine at relatively low pressures is condensed to form an aqueous ammonium carbamate solution, which is thereafter pumped up to the pressure required for urea synthesis. The high-pressure ammonium carbamate solution is subsequently heated to at least a temperature required for the urea reaction using, for instance, steam. Where no separate feeds of ammonia and carbon dioxide are supplied to the urea reactor in that process, the heat required for the endothermic conversion of ammonium carbamate to urea must be introduced into the reactor from an external source. According to the patent disclosure, this is preferably accomplished by overheating the carbamate feed to a temperature about 5.degree. to 25.degree. C. higher than the desired outlet temperature of the urea reactor. Alternatively, the heat for the urea reaction can be provided by external heating of the reactor, for instance by means of heating coils through which high-pressure steam is passed.
This known process has certain disadvantages which derive from the requirement that heat be introduced into the reactor to provide the heat necessary for the conversion of ammonium carbamate to urea. If the required heat for the urea reaction is introduced by means of a heat exchanger in the feed line to the reactor, problems are encountered with respect to the degree of conversion of the carbamate to urea and water, and/or corrosion of the equipment involved. For proper conversion to occur, the temperature selected should be as high as possible. However, since heat is required for the urea reaction, the temperature within the reactor will drop relative to the temperature of the feed to a less than optimum level. This will be detrimental to the conversion of carbamate into urea. Although it is possible to compensate for this temperature decrease by using a higher than usual urea synthesis feed inlet temperature, this poses problems of increased equipment corrosion. At the higher temperatures then required, such expensive corrosion resistant materials must be used that the investment will approach a prohibitively high level.
The alternative is to install heating elements directly in the reactor. However, this poses very difficult problems with corrosion as well.